Reference signal generating method and circuit for differential evaluation of the content of non-volatile memory cells

Abstract: To reduce the supply voltage (V CC ) of a nonvolatile memory (48), a read reference signal (H) is generated having a reference threshold value ranging between the maximum permissible threshold value for erased cells and the minimum permissible threshold value for written cells. To avoid reducing the maximum supply voltage, the characteristic (H) of the read reference signal is composed of two portions: a first portion (H1), ranging between the threshold value and a predetermined value (V s ), presents a slope lower than that of the characteristic (A, G) of the memory cells (50); and a second portion (H2), as of the predetermined value of the supply voltage, presents the same slope as the memory cells. The shifted-threshold, two-slope characteristic is achieved by means of virgin cells (11-13) so biased as to see bias voltages lower than the supply voltage.

Description: The present invention relates to a reference signal generating method and circuit for differential evaluation of the content of nonvolatile memory, in particular flash-EEPROM memory, cells. As is known, comparison reading of the content of a memory cell is performed by comparing a quantity related to the current through the cell to be read with a similar electric quantity related to the current through a reference cell whose content is known. In the case of flash-EEPROM arrays, a similar comparison is frequently also made after erasing the array or part of it, to verify correct erasure and, if necessary, complete it. According to one solution widely used at present, to safely discriminate between the erased and written cells, the reference cell, when biased to a predetermined read voltage, generates a reference current of a value between that of an erased cell and a written cell. For this purpose, an erased reference cell with a threshold equal to the typical threshold is used, and the reference cell circuitry is so designed that the characteristic of the reference cell presents a lower slope than that of the array cells. In the above solution, the minimum evaluation (read or verify) voltage is the value at which the current in the erased cell having the maximum permissible threshold is nevertheless higher than the current in the reference cell. This concept is illustrated in Figure 1 which shows the typical characteristic of a UV, i.e. UV erased, array cell whose threshold depends solely on the fabrication process (curve A); the characteristic of the erased cell with the maximum permissible threshold (curve B); the characteristic of the reference cell (curve C); and the erase-verify voltage (line D), i.e. the voltage at which erase testing is performed to determine correct erasure of the cells (at the erase-verify voltage, cells with a current I ds greater than the corresponding reference cell are considered erased). As can be seen, the erase-verify voltage is slightly higher than the value at which the characteristic of the erased cell with the maximum threshold intersects the reference characteristic. According to the above known solution, the erase-verify voltage also represents the minimum read voltage and hence the minimum supply voltage of the memory (which, for economy, is made equal to the read voltage). In the typical case shown, wherein the erased cell and the reference cell present a 2 V threshold and the erase-verify voltage equals 3.2 V, the maximum-threshold erased cell presents a threshold voltage of 2.5 V, and the minimum supply voltage also equals 3.2 V. On the other hand, current demand is for a reduction in supply voltage as far as possible, to also extend application of this type of memory to portable equipment and low-power applications. A reduction in supply voltage, however, cannot be achieved by simply reducing the erase-verify voltage (and hence shifting to the left the characteristic of the erased cell with the maximum permissible threshold), as this would also result in a leftward shift (towards lower values) of the bell describing the threshold voltage distribution of the array cells when erased. Leftward shifting of the bell would also result in the threshold of the more erased cells (left limit of the bell) being brought too close to zero or even to a negative value, thus resulting in depleted cells, i.e. cells which conduct current even when V GS = 0, and which would therefore be read even when not addressed, thus impairing the accuracy of the reading. Provision must also be made for eliminating as far as possible the presence of cells not yet depleted but with a very low threshold (ranging between 0 and 1 V) which may be subject to hole injection during subsequent programming, thus impairing the oxide layer and hence the reliability of the array. To at least partially solve the problem of depleted or very low threshold cells, processes are currently employed for recovering the depleted cells. In addition to increasing actual erasing time, however, reducing erase-verify voltage also results in an increase in the time taken to recover the depleted cells, thus considerably increasing erasing time as a whole and reducing the speed of the memory. EP-A-0 487 808 discloses a method for generating a reference signal having the features of the preamble of claim 1. Here, the two-slopes read reference signal characteristic is realized by a compensatory programmed cell connected to the reference line. EP-A0 412 837 discloses a method for generating a reference signal wherein the entire characteristic of the read reference signal has a slope lower than the characteristic of the memory cells. The reference threshold value is not shown. It is an object of the present invention to provide a method and circuit permitting a reduction in supply voltage, without impairing reading or erase verify of the array cells or the reliability of the array itself. According to the present invention, there are provided a reference signal generating method and circuit for differential evaluation of the content of nonvolatile memory cells, as claimed respectively in Claims 1 and 7. Preferred non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows the plot of the characteristics used for evaluating memory cells according to a known method; Figure 2 shows a similar plot of the characteristics used for evaluating memory cells according to a first embodiment of the present invention; Figure 3 shows a similar plot according to a second embodiment of the present invention; Figure 4 shows a reference signal generating circuit in accordance with the present invention and corresponding to the Figure 3 diagram; Figure 5 shows a plot of a number of characteristics obtainable as a function of supply voltage at predetermined points of the Figure 4 circuit. The Figure 2 plot shows the characteristic of the read and erase-verify reference voltage according to a first embodiment of the present invention. As shown in Figure 2, the reference characteristic, generated by the reference branch of the array and indicated E, is shifted with respect to the corresponding currently used characteristic (curve C in Figure 1) so that its threshold (voltage at which current I ds is zero) is higher or at least the same as that of the erased cell with the maximum permissible threshold (curve B). For comparison purposes, Figure 2 also shows the characteristic A of a typical UV erased cell. Generation of reference characteristic E provides for reading and verifying the memory cells at a lower voltage as compared with known solutions, and hence for designing memories with a lower supply voltage. In fact, assuming the threshold of the erased cell with the maximum permissible threshold is 2.5 V and the read circuit is so designed that the reference cell threshold is also 2.5 V, even allowing for uncertainty as to the supply voltage (±10%), it is possible to work with a nominal supply voltage of no more than 3 V. Even in the event the supply voltage is the minimum permissible (2.7 V), the current through the reference cell at read voltage is lower than the current through any of the erased cells (even that with a minimum threshold, corresponding to characteristic B) and higher than the current through any of the written cells (characteristic F in Figure 2). According to a second embodiment, for safer reading - in view of the narrow margin in the Figure 2 solution between the threshold of the erased cell with the maximum permissible threshold and the minimum read voltage (200 mV, between 2.5 and 2.7 V) - the threshold of the maximum characteristic of the erased cells is reduced slightly, e.g. by 200 mV, to 2.3 V. Though this shifts the overall threshold distribution bell by 200 mV, the threshold reduction is so small as to bring about only a negligible impairment in terms of the percentage of depleted or pre-depleted cells (with a threshold of 0 to 1 V). Moreover, an erase-verify reference characteristic is generated equal to that of a maximum-threshold erased cell (maximum erased cell characteristic) and as indicated by G in Figure 3. According to the above solution, therefore, the erase-verify reference characteristic no longer coincides with the read reference characteristic. According to a further aspect of the method according to the present invention, the read reference characteristic is no longer represented by a single straight line, but by a broken line formed of two segments: a first segment extending between the x axis and a predetermined voltage V s (e.g. supply voltage V CC ); and a second segment as of voltage V s . More specifically, the first segment presents a slope equal to a fraction of that of the array cell characteristics (e.g. 2/5, as in known characteristic C); and the second segment presents a slope equal to that of the array cell characteristics. The read reference characteristic according to the above embodiment is indicated H in Figure 3, and therefore comprises a first segment H1 and a second segment H2. By virtue of being divided into two segments, read reference characteristic H is in no danger of intersecting the characteristic of a written cell (F in Figure 2) even in the event of a high read or supply voltage. This therefore eliminates the need for reducing the maximum supply voltage of devices not requiring it for reasons of economy and in which a high read voltage (e.g. a nominal voltage of over 5 V) is preferable. In fact, without the above change in slope (characteristic E in Figure 2), the maximum read voltage (and hence supply voltage) must be kept below that at which the read reference characteristic and the characteristic of the least written cell intersect. One embodiment of a circuit for generating the read and erase-verify reference characteristics implementing the above aspect of the present invention is shown in Figure 4 and described below. In Figure 4, the generating circuit is indicated as a whole by 10 and comprises three flash type virgin cells 11, 12, 13 external to the memory array. Cells 11-13, which themselves present a threshold of 2 V, are so biased as to present a characteristic shifted with respect to the typical characteristic of a virgin cell, which bias is achieved by means of voltage shift elements 14 and 15 supplied at the input with supply voltage V CC and supplying a shifted voltage at the output. More specifically, voltage shifter 14 presents an output 14a connected to the gate terminal of cell 11 and supplying a shifted voltage of, say, 1 V, so that the gate-source voltage drop V GS11 of cell 11 equals V CC - 1 V, and cell 11 presents a threshold of 3 V (supply voltage value at which cell 11 begins conducting). Voltage shifter 15 in turn presents an output 15a connected to the gate terminals of cells 12 and 13 and supplying a shifted voltage of, say, 300 mV, so that the gate-source voltage drop (V GS12 , V GS13 ) of cells 12 and 13 equals V CC - 0.3 V, and cells 12, 13 both present a threshold of 2.3 V. Cell 11 presents a precharge circuit 17 and a load transistor 18. Precharge circuit 17, of known type, comprises an N-channel MOS transistor 19 with the source terminal connected to the drain terminal of cell 11, the gate terminal connected to the output of an inverter 20, and the drain terminal connected to transistor 18. The input of inverter 20 is connected to the source terminal of transistor 19 to bias the drain terminal of cell 11 to a fixed voltage, e.g. 1 V, in known manner. Between precharge circuit 17 and load transistor 18, there is inserted a switch 21 formed by a P-channel MOS transistor supplied at the gate terminal with a logic signal R for enabling cell 11 in read mode. Load transistor 18 is a PMOS type, is diode-connected, and presents the source terminal connected to V CC supply line 16, and the drain terminal connected to transistor 21. Transistor 18 is a resistive type with a size (width/length) ratio W/L of, say, 10/2, and forms part of a current mirror circuit 22 including a further P-channel MOS transistor 23 the same size as transistor 18. Transistor 23 presents the source terminal connected to supply line 16, the gate terminal connected to the gate terminal of transistor 18, and the drain terminal connected to the source terminal of a PMOS type transistor 26 operating as a switch and which presents the gate terminal supplied with logic signal R, and the drain terminal connected to a node 27. Node 27 is connected to the drain terminal of cell 12 via a precharge circuit 28 similar to circuit 17 and therefore not described in detail. The source terminal of cell 12 is grounded, and node 27 is also connected to the drain terminal of a PMOS type switch transistor 29, the gate terminal of which is supplied with logic signal R, and the source terminal of which is connected to the drain terminal of a resistive type load transistor 30 forming part of a second current mirror circuit 31. More specifically, P type MOS transistor 30 is diode-connected, presents the source terminal connected to supply line 16, presents a smaller W/L ratio than transistors 18, 23, e.g. 10/4, and is therefore more resistive than transistors 18, 23, as explained below. Current mirror circuit 31 also comprises a PMOS transistor 32 of the same size as transistor 30, and with the source terminal connected to supply line 16, the gate terminal connected to the gate terminal of transistor 30, and the drain terminal connected to the source terminal of a PMOS switch transistor 33, the gate terminal of which is supplied with logic signal R, and the drain terminal of which is connected to a precharge circuit 34 similar to circuits 17 and 28. Precharge circuit 34 is interposed between switch transistor 33 and a node 37 which is also connected to the drain terminal of cell 13, the source terminal of which is grounded. Node 37 is also connected to a read reference node 38 via a precharge circuit 39 similar to circuits 17, 28, 34; and node 38 is connected to supply line 16 via a switch transistor 40 and a load transistor 41. More specifically, switch transistor 40 is a PMOS type supplied with logic signal R at the gate terminal; and load transistor 41 is a resistive PMOS type with the same size ratio as transistors 18, 23, e.g. 10/2. Transistor 41 is diode-connected, and presents the drain terminal connected to switch 40 and to its own gate terminal, the source terminal connected to line 16, and the gate terminal connected to node 27 via a PMOS type switch transistor 42, the gate terminal of which is supplied with a logic erase-verify enabling signal V. Figure 4 also shows a branch of a memory array 48 and the comparators for comparing the array data and reference data. More specifically, of the memory array is shown a cell 50 with a grounded source terminal, the gate terminal supplied with a bias voltage GV, and the drain terminal connected to a node 51 via a precharge circuit 52 similar to circuit 17. Node 51 is connected to supply line 16 via a switch transistor 53 and a load transistor 54 similar, also as regards size, to elements 40, 41 of the reference portion of the circuit. Unlike transistor 40, however, switch transistor 53 presents a grounded gate terminal so that it is always on (closed switch). Reference node 38 and array node 51 are connected to respective inputs of a first comparator 55 for comparing the voltages at said nodes in read mode, which voltages depend in known manner on the currents through reference cell 13 and memory cell 50. Reference node 27 and array node 51 are connected to respective inputs of a second comparator 56 for comparing the voltages at said nodes in erase-verify mode. Operation of the Figure 4 circuit will now be described with reference also to Figure 5 which, as circuit 10 may be used for memory arrays operating at various supply voltages, shows the characteristics of the major circuit nodes as a function of supply voltage. As already stated, by virtue of voltage shifters 14 and 15, cells 11, 12, 13 present lower gate-source voltages V GS with respect to supply voltage, but their characteristic also depends on the working load as explained in the following description wherein I 1 is the current through load transistor 18 (and cell 11), I 2 the current through mirror transistor 23, I 3R the current through cell 12 in read mode, I 3V the current through cell 12 in erase-verify mode, I 4 the current through load transistor 30, I 5 the current through mirror transistor 32, I 6 the current through cell 13, and I 7 the current through load transistor 41. In read mode, logic signal R is low and logic signal V high, so that switches 21, 26, 29, 33, 40 are closed (transistors on), switch 42 is open, and, as long as supply voltage V CC is below 2.3 V, cells 11, 12, 13 are off (V GS11 < 1.3 V, V GS12 = V GS13 < 2 V ). Upon supply voltage exceeding 2.3 V, cells 12 and 13 begin conducting, but their characteristic presents a less than theoretical slope due to the loads present. In fact, in the 2.3 to 3 V supply voltage range, cell 11 is off, I 1 and hence I 2 are zero, and cell 12 only conducts the current supplied by load transistor 30 which, being highly resistive, limits the supplied current. More specifically, with the above sizing, the slope of characteristic I 3R is roughly half that of the intrinsic characteristic of cells 11-13 (shown by I v in Figure 5), so that I 4 = I 3R and I 5 = I 4 due to transistors 30, 32 forming a 1:1 mirror. Within the same supply voltage range, current I 6 through cell 13 presents the intrinsic curve, as shown in Figure 5, due to cell 13 being supplied by both load transistors 32 and 41; and current I 7 through load transistor 41, the value of which equals I 6 - I 5 , presents a slope equal to half the intrinsic slope of cells 11, 12, 13, as shown in Figure 5. With a supply voltage of over 3 V, circuit 10 operates differently. In fact, cell 11 is turned on by virtue of presenting a V GS voltage of over 2 V, and draws a current I 1 with the curve shown in Figure 5, i.e. with a 3 V threshold and the same slope as a virgin cell with no current limitation (I v ). Due to the 1:1 ratio of mirror 22, I 2 = I 1 , and at this point the current through cell 12 is no longer limited by the high resistivity of load transistor 30 by virtue of cell 12 also seeing load transistor 23. Consequently, current I 3R increases alongside the supply voltage with a slope equal to the intrinsic slope, and I 4 (the value of which equals I 3R - I 2 ) presents a constant curve, as does I 5 which equals I 4 . Current I 6 maintains the curve governed by the shifted characteristic of cell 13, while I 7 , the value of which still equals I 6 - I 5 , in this case presents the same slope as a virgin cell. In the example shown, I 3R = I 7 due to the specific sizing of load transistors 18, 23, 30, 32, 41, though in general this does not occur. In known manner, the voltages at nodes 38 and 51 depend solely on currents I 7 and I M (the current in cell 50 with characteristic A or F in Figure 2, depending on whether cell 50 is erased or written) and may therefore be compared directly to safely determine the state of cell 50 even in the presence of low supply voltages. Conversely, in erase-verify mode, logic signal R is high and signal V low, so that switches 21, 26, 29, 33, 40 are open and switch 42 closed, thus connecting node 27 directly to the drain terminal of load transistor 41. In this condition, cell 12 operates like a virgin cell with a shifted characteristic (2.3 V threshold) and unlimited slope (in this case in fact, at any work point, cell 12 sees the same load 41), so that the characteristic of cell 12 (I 3V ) coincides with characteristic I 6 in Figure 5. As such, comparator 56 may compare the voltage at node 27 (again related solely to current I 3V ) with the voltage at node 51 to determine whether cell 50 is sufficiently erased. The advantages of the method and circuit described are as follows. Firstly, they provide for reading and erase-verifying the memory cells even in the presence of low supply voltage, in particular as low as a nominal voltage of 3 V; and, when the erase threshold is shifted slightly downwards, the method according to the invention provides for a good safety margin, thus ensuring highly reliable reading of the cells even with a low supply voltage. Moreover, providing a read reference characteristic with two slopes - a lower slope in the low-voltage range followed by a higher slope - enables use of the same method and circuit for high-voltage storage devices as well, so that the same structure may be used for different applications, thus reducing design, fabrication and storage costs. The circuit described is also extremely straightforward and reliable. In particular, the use of virgin cells with a characteristic shifted by means of voltage shifters is especially advantageous, in that virgin cells provide for a high degree of even long-term precision and hence a high degree of stability of the circuit, unlike, for example, the use of even only partly written cells which, by nature, tend eventually to discharge and so present an intrinsic characteristic drift. Moreover, virgin cells present an intrinsically "safe" threshold involving no uncertainty (unlike, for example, erased cells which, as explained in the beginning, present a statistic distribution of the threshold value). The use of virgin cells therefore provides for eliminating or at least simplifying complex, high-cost matching of the reference cell threshold. Clearly, changes may be made to the method and circuit as described and illustrated herein without, however, departing from the scope of the present invention. In particular, the circuit arrangement of the bias branches of cells 11-13 is purely indicative and may be modified in various respects. For example, as opposed to providing two comparators - one each for read and erase-verify - one comparator, e.g. 55, may be provided by connecting its reference input also to node 27 via a switch element. The sizes and voltage values are also purely indicative. Finally, as opposed to using voltage shifters at the gate terminal of the reference cells for shifting the characteristic of the virgin memory cells, positive voltage offsets may be provided at the source terminals of the cells.

Claims: A reference signal generating method for differential evaluation of the content of nonvolatile memory (48) cells (50); comprising the step of generating a read reference signal (E, H), the characteristic of which presents a slope lower, at least in one portion, than that of the characteristic (A, B, G) of the cells of said memory, characterized in that said read reference signal (E, H) has a reference threshold value ranging between a maximum permissible threshold value for erased cells and a minimum permissible threshold value for written cells. A method as claimed in Claim 1, characterized in that said step of generating a read reference signal comprises the steps of using a virgin cell (12, 13) and shifting its characteristic to said reference threshold value. A method as claimed in Claim 2, characterized in that said step of shifting said characteristic comprises the step of generating, between two terminals of said virgin cell (12, 13), a bias voltage lower than the read voltage of the cells (50) of said nonvolatile memory (48). A method as claimed in one of the foregoing Claims from 1 to 3, characterized in that it also comprises the step of generating an erase-verify reference signal (G) having a characteristic equal to that of an erased cell with a maximum permissible threshold. A method as claimed in one of the foregoing Claims from 1 to 4, for a memory array (48) supplied with a supply voltage; characterized in that said step of generating a reference signal (H) comprises the steps of biasing a reference cell (12, 13), having said reference threshold value. with a voltage related to said supply voltage; limiting the current (I 3R ) through said reference cell when said supply voltage presents a value ranging between said reference threshold value and a predetermined value (V S ) greater than said reference threshold value, to generate a first characteristic portion (H1) with a lower slope than the characteristic of the cells (50) of said memory; and supplying said reference cell with a current of such a value as to generate a second characteristic portion (H2) blending with and having a higher slope than said first portion, when said supply voltage presents a value greater than said predetermined value. A method as claimed in Claim 5, characterized in that said second characteristic portion (H2) presents a slope equal to that of the characteristic (A) of the cells (50) of said memory (48). A reference signal generating circuit (10) for differential evaluation of the content of nonvolatile memory cells; comprising means (15) for generating a read reference signal, the characteristic (E, H) of which presents a slope lower, at least in one portion, than that of the characteristic (A) of the cells (50) of said memory (48), characterised by means (15) for generating a reference threshold value, for said reference signals, which ranges between a maximum permissible threshold value for erased cells and a minimum permissible threshold value for written cells. A circuit as claimed in Claim 7, characterized in that it comprises at least one virgin cell (12, 13); and characteristic shifting means (15) connected to said virgin cell, for shifting the characteristic (E, H) of said virgin cell to said reference threshold value. A circuit as claimed in Claim 8, characterized in that said characteristic shifting means comprise a voltage shifter (15) for generating, between two terminals of said virgin cell (12, 13), a bias voltage lower than the read voltage of the cells (50) of said nonvolatile memory (48). A circuit as claimed in one of the foregoing Claims from 7 to 9, characterized in that it comprises verify source means (12, 41, 42) for generating an erase-verify reference signal (G) having a characteristic equal to that of an erased cell (50) with a maximum permissible threshold. A circuit as claimed in one of the foregoing Claims from 7 to 10, for a memory array (48) supplied with a supply voltage (V CC ); said circuit (10) comprising a first reference cell (12) having said reference threshold value; and means (15) for biasing said reference cell with a bias voltage related to said supply voltage; characterized in that it comprises current limiting means (31) for limiting the current (I 3R ) through said first reference cell, when said supply voltage presents a value ranging between said reference threshold value and a predetermined value (V s ) greater than said reference threshold value, according to a first characteristic portion (H1) having a lower slope than the characteristic of the cells (50) of said memory; and current source means (22) supplying said first reference cell so that the current (I 3R ) in said reference cell follows a second characteristic portion (H2) blending with and having a higher slope than said first portion, when said supply voltage presents a value greater than said predetermined value. A circuit as claimed in Claim 11, characterized in that said first reference cell (12) is a virgin cell; and said biasing means comprise a voltage shifter (15) connected between a supply line (16) and a control terminal of said first reference cell, for generating for said control terminal a first bias voltage lower than said supply voltage (V CC ). A circuit as claimed in Claim 12, characterized in that said current source means comprise a first load circuit (22) connected between said supply line (16) and said reference cell (12); and means (11, 14, 17) for enabling said load circuit when said supply voltage is greater than said predetermined value (V s ); and said current limiting means comprise a second load circuit (31) connected parallel to and presenting a higher resistance with respect to said first load circuit. A circuit as claimed in Claim 13, characterized in that said second load circuit (31) comprises a first MOS transistor (30) having a first width/length ratio; and said first load circuit (22) presents a second MOS transistor (23) having a second width/length ratio smaller than said first ratio. A circuit as claimed in Claim 13 or 14, characterized in that said enabling means (11, 14, 17) comprise a second virgin reference cell (11) having a control terminal; and a second voltage shifter (14) connected between said supply line (16) and said control terminal of said second reference cell, for generating a second bias voltage for said control terminal of said second reference cell; said second bias voltage being lower than said supply voltage and said first bias voltage. A circuit as claimed in one of the foregoing Claims from 13 to 15, characterized in that it comprises a third load circuit (41) connected parallel to said first and second load circuits (22, 31) and of the same resistance as said first load means (22); first enabling means (21, 26, 29, 33) connected to said first and second load circuits, for enabling said first and second load circuits in memory read mode; and second enabling means (42) connected to said third load circuit, for enabling said third load circuit in erase-verify mode.

Inventors: CAMPARDO GIOVANNI, DALLABORA MARCO, CAMPARDO, GIOVANNI, DALLABORA, MARCO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: G11C16
IPC codes: G11C16, G11C17